This application claims priority under 35 U.S.C. xc2xa7119 from Japanese Patent Application No. 10-057992, which has a priority date of Mar. 10, 1998. This application is incorporated by reference herein in its entirety.
The present invention relates to a method for appraising the condition of a polishing cloth used to polish semiconductor wafers, and a method for making semiconductor wafers using a polishing cloth so appraised. In particular, the invention relates to ensuring the cleanliness of semiconductor wafers produced using a polishing cloth.
The quality of a semiconductor wafer that is polished with a polishing cloth is specified by the shape quality and the cleanliness of the surface thereof. The shape quality is evaluated using the micro-roughness measurement, such as the peak-to-valley (P-V) measurement, or by a measurement of uneven thickness (e.g., Total Thickness Variation (TTV) or (LTV)). The cleanliness of the surface is determined by counting the number of particles on the surface. The diameter of particles to be counted becomes smaller year by year due to the increasingly stringent demands of industry, making them increasingly difficult to distinguish from crystal original pits (COP). Therefore, in practice, particles are counted as part of a light point defect (LPD) number, in which particles and COP are counted together. In the LPD measurement technique, the surface of the semiconductor wafer is irradiated with laser light. Where a particle or COP exists, the reflected light from the laser is scattered. The existence of particles or COPs is detected by receivers that measure the scattered light. In the LPD measurement, the particle diameter is preset and the total number of particles and COPs that are the same size or larger than that preset diameter are counted.
However, the measured LPD number is just a measurement result. To obtain better semiconductor wafer quality, factors affecting that quality need to be improved. As a result of the inventor""s studies, it has been discovered that the condition of polishing cloths used to polish semiconductor wafers is one such factor. However, the conventional index used to judge the condition of polishing cloths is based only on a measurement of the polished off thickness. Thus, this conventional method evaluates only the rate at which the polishing cloth polishes the semiconductor wafer. Therefore, under conventional methods, the polishing cloth will not be exchanged so long as it continues to polish off a certain thickness of semiconductor material, even if the LPD number increases. This means that a low LPD number cannot be maintained if the polishing rate of the cloth has not deteriorated. Simply counting the number of times that a polishing cloth has been used does not allow for consistently low LPD numbers to be maintained. The subject matter of the present invention is to provide a method for appraising the condition of a polishing cloth, and to provide a method for manufacturing semiconductor wafers employing the appraisal method, to allow low LPD numbers to consistently be maintained.
The present invention includes a method for appraising the condition of a polishing cloth used to polish semiconductor wafers. The method includes measuring the number of LPDs on the polished surface of the semiconductor wafer after it has been polished using the polishing cloth that is being appraised. In particular, after the wafer has been polished, washed, and dried, the number of LPDs having a certain diameter or greater are counted. The condition of the polishing cloth is appraised by the number of LPDs, this number being a cloth quality index. When the number of LPDs reaches or exceeds a prescribed number, the polishing cloth is exchanged for a fresh one before the next semiconductor wafer is polished. In a preferred embodiment of the present invention, the diameter of particles comprising the LPDs to be counted is greater than about 0.12 xcexcm. In a most preferred embodiment, the diameter of particles comprising the LPDs to be counted is greater than a certain diameter, and that certain diameter is from about 0.12 xcexcm to about 0.16 xcexcm.
The present invention also includes a method for manufacturing semiconductor wafers that are polished using a polishing cloth, while maintaining consistently low LPD numbers. The method includes polishing a semiconductor wafer using a polishing cloth, and determining the number of LPDs having a certain particle size or greater. When the number of LPDs of the specified size or greater meets or exceeds a prescribed number, the polishing cloth is exchanged for a new one. This exchange takes place before the next semiconductor wafer is to be polished. In a preferred embodiment of the invention, the size of particles comprising LPDs is greater than about 0.12 xcexcm. In a most preferred embodiment, the diameter of particles comprising the LPDs to be counted is greater than a certain diameter, and that certain diameter is from about 0.12 xcexcm to about 0.16 xcexcm.